1. Field of the Invention
The present invention concerns an illumination device for illuminating an illumination light at an optional color temperature by emission of LED (Light Emitting Diodes) for each of RGB colors each of which is lit at a light quantity in proportion with a driving current.
2. Statement of the Related Art
In recent production lines, image processing techniques have been utilized frequently for discriminating plural kinds of works, or inspecting the quality of works which are supplied and sent simultaneously in one identical line.
In image processing, an appropriate illumination light is illuminated from an illumination apparatus to a work and images of the work are taken up by an image pick-up device such as a CCD camera. In this case, selection of the illumination light is extremely important and it is necessary to set color and light quantity optimal to the color and the surface property of the work.
For example, when a white light is illuminated to a lustrous work, for example, semiconductor wafers, LCD electrodes, fabricated metal materials, surface mounted solder patterns, blister packs and aluminum foil packages, since reflected light therefrom intrudes in photographed images, the accuracy of the image processing is deteriorated. Further, when an illumination light including infrared light or ultraviolet light is exposed to a work covered with a heat sensitive resin or UV-ray curable resin the portion of the resin undergoes photosensitization.
Further, illumination apparatus capable of illuminating an optional color has been demanded not only in the field of image processing technique described above but also in the lighting design for shop illumination or show windows. Accordingly, an illumination apparatus using LED of three primary colors RGB has been proposed.
In the proposed apparatus, as shown in FIG. 3, three half-mirrors 41R, 41G, and 41B are located on an outgoing optical axis XW, and LED 42R, 42G, and 42B for RGB are arranged each on the mirror optical axis XR, XG and XB, and light control circuits 43R, 43G, and 43B capable of optionally setting the light quantity for each of the LED 42R, 42G, and 42B are connected to them.
According to the apparatus, lights illuminated from LED 42R, 42G, and 42B are passed through the mirror optical axis XR, XG, and XB, reflected on the half-mirror 41R, 41G, and 41B, mixed on the outgoing optical axis XW and outputted from the light outgoing end 44.
In this apparatus, since respective LED 42R, 42G, and 42B can emit lights each at an optional light quantity by the light control circuit 43R, 43G, and 43B, when each of them is illuminated at an equal light quantity, white light can be obtained, and a light can be illuminated while optionally setting the color temperature by varying the ratio of the light quantity.
However, after setting the color temperature of the light illuminated from the light outgoing end 44, it is difficult to increase or decrease the brightness while maintaining the color temperature as it is.
That is, as shown in FIG. 4, since the voltage-light quantity characteristic differs on every color, as well as it is not constant even for the LED of an identical color, even if the voltage supplied to each of them is changed equally, the color temperature can not be maintained constant.
Accordingly, individual adjustment was obliged so far while measuring the intensity for each of the wavelength components contained in the illumination light by using, for example, a spectrophotometer and adjusting operation was extremely troublesome.
In view of the above, the present invention intends for enabling simple control for the brightness while maintaining the color temperature as it is after setting the color temperature for the illumination light.